1. Field of the Invention
The present invention relates to a horizontal centering compensation circuit for a video display apparatus. More specifically, the present invention relates to a circuit for compensating a horizontal centering in a video display apparatus. Further, even when the video display mode of a multi-mode monitor is shifted, the present invention promotes automatically minimizing or preventing any biasing of the picture displayed on a video display apparatus.
2. Description of the Related Art
Recently, in accordance with the progress of computer systems, there have been developed multi-mode monitor systems which are capable of supporting various resolutions.
A multi-mode monitor refers to a monitor in which the monitor has a compatibility with at least two or more video modes. Each time when the supporting mode is shifted in a video cassette, the picture size and position are varied, the horizontal and vertical synchronization are realized, the deflecting section is optimized, and various bias correcting circuits are re-adjusted.
Particularly, in recent times, direct current (dc) currents are added to the deflecting currents which deflect the electron beams in a cathode ray tube, thereby making it possible to variably move the horizontal axis of the picture. A compensation circuit, in this regard, has been generally adopted.
Referring to FIG. 1, FIG. 1 is a circuit diagram illustrating a conventional horizontal centering compensation circuit for adjusting the horizontal center position of the picture. As shown in FIG. 1, the conventional horizontal centering compensation circuit 300 is connected to a horizontal deflecting coil L1 of a horizontal deflection output circuit 100, for adding a magnitude-different and direction-different direct current (dc) current to the deflecting current which flows through the horizontal deflecting coil L1.
As shown in FIG. 1, the horizontal deflection output circuit 100 includes: a horizontal output transistor Q1 that is switched by the output signals of a horizontal driving circuit 10; a damping diode D1 connected in parallel to the horizontal output transistor Q1; a retrace capacitor C1 connected in parallel to the damping diode D1; and the horizontal deflecting coil L1 connected in parallel to the retrace capacitor C1.
Thus, the horizontal output transistor Q1 is turned on or off in accordance with on or off output signals of the horizontal driving circuit 10. Further, the deflecting current which is supplied to the horizontal deflecting coil L1 during the turn-on period of the horizontal output transistor Q1 is charged into the retrace capacitor C1 during the turn-off period of the horizontal output transistor Q1, and then is discharged to the horizontal deflecting coil L1 . Through this procedure, a saw tooth shaped deflecting current is supplied to the horizontal deflecting coil L1. Thus, magnetic fields which are generated from the horizontal deflecting coil L1 deflect the electron beams of a cathode ray tube of a video display apparatus.
The horizontal centering compensation circuit 300 which is connected to the horizontal deflection output circuit 100 includes: a horizontal position adjusting coil L3 connected in parallel to the horizontal deflecting coil L1; and a movable terminal a and first fixed terminal b and a second fixed terminal c of a switch SW1. By the switch SW1, the movable terminal a is connected to the horizontal position adjusting coil L3 and, by the switch SW1, the first fixed terminal b is connected through a first diode D2, a resistor R1 and a coil L2 to a step-up converter 12, while the second fixed terminal c is connected through a second diode D3 and a capacitor C2 to the ground GND. As illustrated in FIG. 1, the first diode D2 and the second diode D3 are connected in the mutually opposite directions.
Therefore, if the axis of the picture is biased to the left side or right side in the high frequency band, then the movable terminal a of the switch SW1 can be connected to the first fixed terminal b or the second fixed terminal c, so as to bring the horizontal axis of the picture to the center. First, if the movable terminal a is connected to the first fixed terminal b, the output signals of the step-up converter 12 are supplied through the coil L2, the resistor R1 and the first diode D2 to the horizontal position adjusting coil L3.
Accordingly, a direct current (dc) current having a magnitude and direction is supplied to the horizontal deflecting coil L1, and the center position of the picture moves in accordance with the direction and degree of the horizontal raster shift which is adjusted by the impedances of the horizontal position adjusting coil L3, the coil L2 and the resistor R1.
Meanwhile, if the movable terminal a of the switch SW1 is connected to the second fixed terminal c, then the current of the horizontal position adjusting coil L3 is grounded through the second diode D3 and the capacitor C2. Accordingly, a direct current (dc) current which has an opposite direction and a different magnitude from the above described direct current (dc) current supplied when the movable terminal a is connected to the first fixed terminal b is supplied to the horizontal deflecting coil L1.
As described above, in the conventional horizontal centering compensation circuit 300, the horizontal axis of the picture moves in opposite directions in accordance with which fixed terminal b or c the movable terminal a is connected. Therefore, the picture which is biased to the left or right can be brought to the center of the screen. However, this conventional horizontal centering compensation circuit of FIG. 1 is for correcting the left and right bias at a radio frequency band. Therefore, when the video display mode is shifted, such as when a video card is replaced with another video card in a computer system, an accurate horizontal centering can become difficult.
An object among other objects of the present invention is to promote overcoming the above described disadvantage of accurate horizontal centering when a video card is replaced in the aforementioned conventional technique.
Therefore it is an object among other objects of the present invention to provide a circuit for compensating a horizontal centering in a video display apparatus, in which even when the video display mode of a multi-mode monitor is shifted, any biasing of the picture can be automatically corrected.
In achieving the above object and other objects of the present invention, a circuit for compensating a horizontal centering in a video display apparatus according to the present invention includes: a control unit for counting a frequency of externally inputted video synchronizing signals to selectively output a first switching control signal and a second switching control signal based on counted result of the frequency of the externally input video synchronizing signals; a horizontal centering compensation circuit for providing a first power having a first voltage level and second power having a second voltage level, the first power and the second power having respective voltage levels and directions different from each other, to selectively output the first power and the second power in response to a respective one of the first switching control signal and the second switching control signal of the control unit; and a horizontal deflection output circuit for receiving a driving power, to generate a magnetic field for deflecting electron beams in response to an externally input horizontal driving signal, so as to variably adjust a horizontal center position of a picture displayed on the video display apparatus in accordance with the selective output of the first power and the second power from the horizontal centering compensation circuit.
In the present invention, it is desirable the control unit, such as a microcomputer or microprocessor, counts a frequency of the video synchronizing signals of a video card of a computer system to check the shifting of display modes.
Further, in the present invention, when the display mode is shifted, the control unit, such as the microcomputer or microprocessor, computes picture centering compensation values, and adjusts the direction and the range of a horizontal raster shift in accordance with the computed picture centering compensation values.
Therefore, in accordance with the present invention, in a multi-mode monitor, even when a video card is replaced, a horizontal centering of the picture can be accurately compensated.